A simple, performant CSV to TSV converter.

csv2tsv.c

// A simple, performant CSV to TSV converter.
// Roughly the same performance as `xsv fmt -t '\t'`
//   if compiled with `-O1` or higher.

// Note: if the input doesn't end with '\n', this program will still
// work correctly but won't add a final '\n'. If it ends with '\n',
// the output will have it.

// Copyright 2020 Noureddin (github.com/noureddin).
// License: Creative Commons Zero (aka Public Domain).

#include <stdio.h>    // getchar, putchar, fprintf
#include <stdbool.h>  // bool, true, false
#include <stdlib.h>   // exit

#define QUOTE  '"'
#define  FS    ','  // input field separator
#define  RS   '\n'  // input record separator
#define OFS   '\t'  // output field separator
#define ORS   '\n'  // output record separator


int main(void) {
    int ch;
    bool quoted;
    bool field_start = true;

#define StartField()  do { putchar(OFS); field_start = true; } while (0)
#define StartRecord() do { putchar(ORS); field_start = true; } while (0)

    while ((ch = getchar()) != EOF) {

        if (field_start) {
            switch (ch) {
                case FS:  StartField();  break;
                case RS:  StartRecord(); break;
                case QUOTE: quoted = true;  field_start = false; break;
                default:    quoted = false; field_start = false; putchar(ch);
            }
            continue;
        }

        // if not field_start
        switch (ch) {
            case FS:
                if (quoted)
                    putchar(ch);
                else
                    StartField();
                break;
            case RS:
                StartRecord();
                break;
            case QUOTE: {
                int ch2 = getchar();
                switch (ch2) {
                    case QUOTE: putchar(QUOTE); break;
                    case FS:    StartField();   break;
                    case RS:    StartRecord();  break;
                    case EOF:   return 0;
                    default:
                        fprintf(stderr, "Unexpected sequence: %c%c\n", ch, ch2);
                        exit(2);
                }
                break;
            }
            default:
                putchar(ch);
        }

    }

#undef StartField
#undef StartRecord

}